1. Field of the Invention
This invention relates to the field of switching circuits and, more particularly, for switching circuits for advanced digital radio receivers and transmitters.
2. Description of the Prior Art
A simple classic radio receiver consists of a single antenna coupled to a downconverter that extracts a single “baseband” channel. In a modern receiver, the baseband signal is digitized with an analog-to-digital converter (ADC) and further processed in the digital domain. A classic transmitter contains essentially the same components working in the reverse direction; a transceiver contains both a transmitter and a receiver packaged together.
A multi-band, multi-channel RF communications system (see FIG. 1a) can include multiple antennas, and can extract multiple baseband channels or groups of channels simultaneously. This requires a switch matrix, which permits distribution of various signals between each antenna (corresponding to each band) and one or more appropriate radio receiver channels. If the system is to be flexible and reconfigurable, it should be possible to redirect input signals to selected output channels.
The signal at the antenna is an analog waveform, even if it may be encoding a digital signal. In a conventional receiver system of the prior art, as illustrated in FIG. 1a, both the switch matrix and the channelizing receivers are analog components, and similarly for the transmitter. However, these band-specific precision analog components are often expensive and limited in their flexibility and reconfigurability. Furthermore, these analog switching systems have severe deficiencies in terms of losses, isolation, crosstalk, and ability to multicast.
For these reasons, the communications industry would like to move toward an approach known as “software-defined radio” (SDR) or “software radio”, where all data processing is carried out in the digital domain, except right at the antenna itself. This requires ultrafast data converters, with sampling rates of tens of GHz and excellent linearity. ADCs with the requisite properties have recently been demonstrated, based on superconductor electronics using Josephson junctions, with circuit designs based on rapid-single-flux-quantum logic (RSFQ). It is natural that this data conversion be carried out right at the antenna, as illustrated in FIG. 1b. But in this case, the switching must also be carried out directly on the digital-RF signals. Furthermore, the precision and linearity of these signals can be maintained in the distribution network only if the sampling clock is distributed along with the data bits. This requires a new type of digital-RF switch matrix, which has not been reported before, and which is the subject of the present invention.
Furthermore, the digital-RF transceiver architecture allows natural partitioning between band-specific (analog) and band-independent (digital) components. Analog components, such as antennas and amplifiers are optimized for performance within a particular frequency band. Even data converters between analog and digital formats, ADCs and DACs, work best with designs that target specific frequency bands. Furthermore, an ADC or DAC optimized for a particular frequency band will typically have a particular sampling frequency (clock frequency fclock) that is preferred for best performance. For example, a radio-frequency bandpass ADC designed for a center frequency f) may exhibit the greatest dynamic range for a sampling frequency that is four times the center frequency (fclock=4×f)). On the contrary, digital signal processing units, operating on numbers, are independent of the signal characteristics. This partitioning enables the true software radio paradigm by allowing full software programmability of the RF distribution network. Superconductor electronics are fast enough to digitize at multi-GHz RF and perform subsequent processing completely in the digital domain.
Switch matrices based on superconducting electronic circuits have been recently reported by several inventors. For example, see (1) U.S. Pat. No. 6,960,929, issued Nov. 1, 2005 by inventor Fernand D. Bedard, entitled Superconductive Crossbar Switch, (2) U.S. Pat. No. 6,917,537, issued Jul. 12, 2005 by inventor Paul I. Bunyk entitled RSFQ Batcher-Banyan Switching Network, (3) U.S. Pat. No. 6,865,639, issued Mar. 8, 2005 by inventor Quentin P. Herr entitled Scalable Self-Routing Superconductor Switch, and (4) Hashimoto et al., Implementation of a 4×4 Switch With Passive Interconnects, IEEE Trans. Appl. Supercon., vol 15, no. 2, June 2005, pp. 356-359.
However, none of these patents was designed for an application in RF communications, and none of these include switches which route the clock signal together with the data signal, which are two of several reasons this invention distinguishes over the prior art. See also the article by D. K. Brock, O. A. Mukhanov, and J. Rosa, “Superconductor Digital Development for Software Radio,” IEEE Commun. Mag., pp. 174-179, February 2001, and K. K. Likharev and V. K. Semenov, “RSFQ Logic/Memory Family: A new Josephson junction technology for sub-THz digital systems”, IEEE Trans. Appl. Supercond., vol. 1, pp. 3-28, 1991.
3. Problems of the Prior Art
The prior art switches have been expensive and limited in their flexibility and ability to reconfigure. In addition, they have severe deficiencies in terms of losses, isolation, cross talk and ability to multicast.
It is natural and desirable that data conversion be carried out right at the antenna, but, in such a case, the switching must also be carried out directly on the digital-RF signals. Further, the precision and linearity of these signals can be maintained in the distribution network only if the sampling clock is distributed along with the data bits. This requires and new type of digital-RF switch matrix which is the subject of the present invention.